Plasma processing devices for plasma processing such as film formation by, for example, plasma CVD (Chemical Vapor Deposition) has been widely known. Such plasma processing devices are used for forming films, such as interlayer insulating films, passivation films, gate insulating films, semiconductor films, in processing semiconductor wafers or glass substrates used for LCD (Liquid Crystal Display).
Especially, parallel plate plasma enhanced CVD systems have been known as systems excellent in their convenience and operability (e.g., Japanese Patent Application Laid Open Publication No. 11-31685A). Here, a general parallel plate plasma CVD system is described with reference to a perspective view of FIG. 3 and a sectional view of FIG. 4 each showing the main part thereof.
The parallel plate plasma processing device includes, as shown in FIG. 4, a processing chamber (film deposition chamber) 5, which is a vacuum vessel, and a discharge electrode 2, in which two metal plates are disposed in parallel to each other in the processing chamber 5. A vacuum pump 10 is connected to the processing chamber 5.
The discharge electrode 2 is composed of a cathode electrode 2a fixed and supported to an electrode supporter 22 in the processing chamber 5 and an anode electrode 2b facing the cathode electrode 2a. 
The anode electrode 2b is grounded electrically, and a target substrate 4, which is a target made of silicon, glass or the like to be plasma processed, is mounted thereon.
A plurality of gas introducing holes 6 are formed in the cathode electrode 2a, as shown in FIG. 3. A material gas supplied from a gas supply section 13 is supplied to the space between the cathode electrode 2a and the anode electrode 2b through the gas introducing holes 6.
A RF generator 1 for applying voltage for generating plasma 11 is connected to the cathode electrode 2a via a matching circuit 16. Usually, the RF generator 1 uses electric energy of radiofrequency at 13.56 MHz, for example.
The matching circuit 16 is provided for matching impedance of the RF generator 1 to impedance of the cathode electrode 2a. Specifically, the matching circuit 16 is composed of a plurality of variable capacitors 7, 8 and a coil 9, as shown in enlarged scale in FIG. 5. The matching circuit 16 detects incident power to become incident to the cathode electrode 2a and reflected power reflected to the RF generator 1 and controls, based on the detected values, the variable capacitors 7, 8 so that the incident power and the reflected power become maximum and minimum, respectively. Whereby, power output from the RF generator 1 is effectively used.
The RF generator 1 and the matching circuit 16 are driven, a given voltage is applied to the cathode electrode 2a, and the material gas is introduced into the space between the cathode electrode 2a and the anode electrode 2b from the gas supply section 13 through the gas introducing holes 6.
Accordingly, an electric field is generated between the two plates of the discharge electrodes 2 to generate plasma 11, which is a glow discharge phenomenon by a dielectric breakdown phenomenon of the electric filed. Electrons in the plasma 11 accelerated in the vicinity of the cathode electrode 2a promote dissociation of the material gas to generate radial. The radical disperses toward the target substrate 4 mounted on the anode electrode 2b having a grounded potential, as shown by an arrow R in FIG. 4, and deposits on the surface of the target substrate 4, thereby forming a film.
Problems to be Solved
In plasma processing, it is inevitable that an unnecessary film is deposited on the surface of the discharge electrode and the inner wall of the processing chamber, as well as on the target substrate. This changes the impedance of the discharge electrode.
As a countermeasure, it is possible that the matching circuit maintains the maximum incident power by changing the capacitance of the variable capacitors according to the change in impedance of the discharge electrode. However, the net incident power to become incident to the discharge electrode changes in association with the change in impedance of the discharge electrode, and therefore, incident power required for precise film formation cannot be obtained. As a result, no desirable film thickness may be obtained or variation in film quality may be caused. The film quality may be evaluated with hydrogen concentration contained in, for example, a SiNx film.
For tackling this problem, it is considered that the film formation rate and the film quality are checked visually at given periods (e.g., a day) and the power of the RF generator and the time period for film formation are adjusted according to the checked result so as to prevent the film from being changed in thickness and from lowering in quality. However, such fine adjustment is difficult and involves much labor.
For solving this problem, it is possible to remove such an unnecessary film by periodically self-cleaning the inside of the processing chamber. In the self cleaning, an etching gas such as NF3 is introduced into the processing chamber and free radical is generated by plasma excitation, thereby removing an unnecessary film with the use of the thus generated free radical. However, the impedance of the discharge electrode is different between before and after the cleaning, and this makes it difficult to maintain the film quality.